1. Field of the Invention
The present invention relates to an acoustic wave filter device including a longitudinally coupled resonator-type acoustic wave filter unit, in which at least three IDTs are provided on a piezoelectric substrate. More particularly, the present invention relates to a longitudinally coupled resonator-type acoustic wave filter device having a balance-unbalance conversion function.
2. Description of the Related Art
Surface acoustic wave filter devices having a balance-unbalance conversion function have been used in band filters of mobile phones, and other devices. Furthermore, in recent years, in place of surface acoustic wave filter devices using a surface acoustic wave, boundary acoustic wave filter devices using a boundary acoustic wave have attracted attention. When acoustic wave filter devices, such as surface acoustic wave filter devices or boundary acoustic wave filter devices, have a balance-unbalance conversion function, a separate balance-unbalance conversion function element called a balun can be omitted. As a result, it is possible to reduce the number of parts, and it is possible to decrease the size of mobile phones.
In Japanese Unexamined Patent Application Publication No. 2003-124782 described below, a surface acoustic wave filter device having a balance-unbalance conversion function is disclosed. As shown in FIG. 19, in a surface acoustic wave filter device 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2003-124782, a surface acoustic wave filter element chip 1003 is housed in a package 1002 having an upward facing opening 1002a. In the surface acoustic wave filter element chip 1003, on a piezoelectric substrate 1004, an electrode configuration shown in FIG. 19 is provided, and a surface acoustic wave filter having a balance-unbalance conversion function is provided.
That is, a 3-IDT-type longitudinally coupled resonator-type surface acoustic wave filter unit 1008 is connected to an unbalanced terminal 1005 via surface acoustic wave resonators 1006 and 1007. Here, IDTs connected to the unbalanced terminal 1005 include a first IDT 1009 and a third 1011. Then, a second IDT 1010 is arranged so as to be interposed between the first IDT 1009 and the third IDT 1011. One end of the second IDT 1010 is connected to the first balanced terminal 1012, and the other end thereof is connected to the second balanced terminal 1013.
The other end of the IDT 1009 connected to the unbalanced terminal 1005 is connected to an electrode pad 1015 connected to the ground potential via a ground wiring pattern 1014. Similarly, the other end of the IDT 1011, one end of which is connected to the unbalanced terminal 1005, is connected to an electrode pad 1017 connected to the ground potential through a ground wiring pattern 1016.
On the other hand, in Japanese Patent No. 3918102, an acoustic wave filter device is disclosed in which a first 3-IDT-type longitudinally coupled resonator-type acoustic wave filter unit is connected between an unbalanced terminal and a first balanced terminal, and a second 3-IDT-type longitudinally coupled resonator-type acoustic wave filter unit is connected between an unbalanced terminal and a second balanced terminal, thereby realizing a balance-unbalance conversion function. Here, it is described that by designing ground wiring, the amount of out-of-band attenuation can be reduced.
In the surface acoustic wave filter device 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2003-124782, as described above, by using one longitudinally coupled resonator-type acoustic wave filter unit 1008, a balance-unbalance conversion function is provided. Therefore, not only can a balun be omitted, but an electrode configuration achieving a balance-unbalance conversion function can also be simplified. However, in the surface acoustic wave filter device 1001 disclosed in Japanese Unexamined Patent Application Publication No. 2003-124782, there is a problem in that the amount of attenuation outside the passband is insufficient.
On the other hand, in Japanese Patent No. 3918102, a configuration is shown in which by designing ground wiring, a lower loss is achieved. However, in Japanese Patent No. 3918102, the configuration in which, by designing ground wiring, the amount of out-of-band attenuation is reduced, is effective only for a filter device having the specific electrode configuration disclosed in Japanese Patent No. 3918102.